Temperature-compensated piezoelectric oscillator

ABSTRACT

A temperature-compensated piezoelectric oscillator includes a voltage-controlled piezoelectric oscillator, a first frequency divider, a second frequency divider, and a controller. The voltage-controlled piezoelectric oscillator has an output frequency controlled by a control voltage and oscillates the output frequency having a center frequency f 0  at an ambient temperature of t 0 . The first frequency divider frequency-divides the output frequency from the voltage-controlled piezoelectric oscillator by N to output a first divided output frequency. The second frequency divider frequency-divides the output frequency from the voltage-controlled piezoelectric oscillator by M to output a second divided output frequency. The controller controls the control voltage to be applied to the voltage-controlled piezoelectric oscillator on the basis of a frequency difference between the first and second divided output frequencies upon a variation in ambient temperature of t 0  to keep the output frequency of the voltage-controlled piezoelectric oscillator at the center frequency f 0 .

BACKGROUND OF THE INVENTION

The present invention relates to a temperature-compensated piezoelectricoscillator which oscillates a constant output frequency regardless ofvariations in ambient temperature.

FIG. 2 shows a conventional digital temperature-compensatedpiezoelectric oscillator (microcomputer-controlled piezoelectricoscillator). This temperature-compensated piezoelectric oscillatorcomprises a temperature sensor 9, an A/D converter 5 for converting ananalog output from the temperature sensor 9 into a digital code tooutput it, a microcomputer circuit 6 and a memory circuit 7 whichreceive the digital code output from the A/D converter 5, a D/Aconvertor 8 for converting an output digital code from the microcomputercircuit 6 into an analog voltage to output it, and a voltage-controlledpiezoelectric oscillator 1 for receiving the analog voltage output fromthe D/A convertor 8.

In the temperature-compensated piezoelectric oscillator having thisarrangement, the temperature sensor 9 detects an ambient temperature Tof the voltage-controlled piezoelectric oscillator 1 as an analogvoltage. The A/D converter 5 converts the detected ambient temperature Tinto a digital code to output it to the microcomputer circuit 6 and thememory circuit 7. The microcomputer circuit 6 calculates compensationdata for the ambient temperature T at that time on the basis of aprogram and constant which are prestored in the memory circuit 7 tocompensate the frequency/temperature characteristics of thevoltage-controlled piezoelectric oscillator 1. The output digital codeis supplied to the D/A convertor 8. The D/A convertor 8 converts theoutput digital code from the microcomputer circuit 6 into an analogvoltage to supply it to the voltage-controlled piezoelectric oscillator1.

With this arrangement, an output frequency F from the voltage-controlledpiezoelectric oscillator 1 can always be kept constant regardless of achange in ambient temperature T.

In this conventional temperature-compensated piezoelectric oscillator,however, when the ambient temperature T abruptly changes, a temperaturedifference occurs between a piezoelectric element in thevoltage-controlled piezoelectric oscillator 1 and the temperature sensor9. For this reason, accurate temperature compensation is not performedto degrade frequency stability.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide atemperature-compensated piezoelectric oscillator in which accuratetemperature compensation is performed even with an abrupt change inambient temperature not to affect frequency stability.

In order to achieve the above object, according to the presentinvention, there is provided a temperature-compensated piezoelectricoscillator comprising voltage-controlled piezoelectric oscillationmeans, having an output frequency controlled by a control voltage, foroscillating the output frequency having a center frequency f₀ at anambient temperature of t₀, first frequency division means forfrequency-dividing the output frequency from the voltage-controlledpiezoelectric oscillation means by N to output a first divided outputfrequency, second frequency division means for frequency-dividing theoutput frequency from the voltage-controlled piezoelectric oscillationmeans by M to output a second divided output frequency, and controlmeans for controlling the control voltage to be applied to thevoltage-controlled piezoelectric oscillation means on the basis of afrequency difference between the first and second divided outputfrequencies upon a variation in ambient temperature of t₀ to keep theoutput frequency of the voltage-controlled piezoelectric oscillationmeans at the center frequency f₀.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a temperature-compensatedpiezoelectric oscillator according to an embodiment of the presentinvention; and

FIG. 2 is a block diagram showing a conventional temperature-compensatedpiezoelectric oscillator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described below indetail. FIG. 1 shows a temperature-compensated piezoelectric oscillatoraccording to the embodiment of the present invention.

The temperature-compensated piezoelectric oscillator of this embodimentcomprises a first frequency divider 12 for frequency-dividing an outputfrequency F from a voltage-controlled piezoelectric oscillator 11 by Nto obtain a first divided output frequency F1, a second frequencydivider 13 for frequency-dividing the output frequency F from thevoltage-controlled piezoelectric oscillator 11 by M to obtain a seconddivided output frequency F2, and a high-precision frequency comparator14 for outputting an analog voltage in accordance with a frequencydifference between the first and second divided output frequencies F1and F2. The analog voltage output from the high-precision frequencycomparator 14 is applied to an A/D convertor 15.

An output frequency F(t₀) from the voltage-controlled piezoelectricoscillator 11 at an ambient temperature T of t₀ is defined as a centerfrequency f₀. A frequency difference between the first and seconddivided output frequencies F1(t₀) and F2(t₀) at that time is prestoredas a reference value in a memory circuit 17.

More specifically, the output frequency F from the voltage-controlledpiezoelectric oscillator 11 at the ambient temperature T of t₀ isdefined as F(t₀)=f₀, and f₀ is defined as the center frequency of thevoltage-controlled piezoelectric oscillator 11. In this case, the firstdivided output frequency F1 of the frequency divider 12 is

    F1(t.sub.0)=f.sub.0 /N

and, the second divided output frequency F2 of the frequency divider 13is

    F2(t.sub.0)=f.sub.0 /M

The first and second divided output frequencies F1(t₀) and F2(t₀) areinput to the high-precision frequency comparator 14.

A frequency difference D(t₀) between the first and second divided outputfrequencies F1(t₀) and F2(t₀) input to the high-precision frequencycomparator 14 is obtained by

    D(t.sub.0)=f.sub.0 /N-f.sub.0 /M

In accordance with this frequency difference D(t₀), the high-precisionfrequency comparator 14 outputs an analog voltage:

    V(t.sub.0)=G{D(t.sub.0)}

This analog voltage V(t₀) is converted by the A/D convertor 15 into adigital code:

    A(t.sub.0)=H{V(t.sub.0)}

The obtained digital code is input to a microcomputer circuit 16 and thememory circuit 17.

The memory circuit 17 stores, as a reference value, the input digitalcode A(t₀), i.e., the frequency difference between the first and seconddivided output frequencies F1(t₀) and F2(t₀) at the ambient temperatureT of t₀ by the microcomputer 16.

Assume that the ambient temperature T is kept at t, and the outputfrequency F from the voltage-controlled piezoelectric oscillator 11 isat f₀. If the ambient temperature T changes from this state to t+Δt, andthe output frequency F from the voltage-controlled piezoelectricoscillator 11 changes to

    F(t+Δt)=f.sub.0 +Δf

the first divided output frequency F1 is

    F1(t+Δt)=(f.sub.0 +Δf)/N

and the second divided output frequency F2 is

    F2(t+Δt)=(f.sub.0 +Δf)/M

The frequency difference D(t+Δt) between the first and second dividedoutput frequencies F1(t+Δt) and F2(t+Δt) is ##EQU1## In accordance withthe frequency difference D(t+Δt), the high-precision frequencycomparator 14 outputs an analog voltage:

    V(t+Δt)=G{D(t.sub.0)+Δfx(1/N-1/M)}

This analog voltage is converted by the A/D convertor 15 into a digitalcode:

    A(t+Δt)=H{V(t+Δt)}

The obtained digital code is input to the microcomputer circuit 16 andthe memory circuit 17.

The microcomputer circuit 16 calculates a change amount Δf of the outputfrequency F from the voltage-controlled piezoelectric oscillator 11 withrespect to the center frequency f₀ on the basis of the reference valueA(t₀) stored in the memory circuit 17 and the digital code A(t+Δt) fromthe A/D convertor 15. Next, the microcomputer circuit 16 calculates ananalog voltage in accordance with this change amount Δf to supply theoutput digital code to a D/A convertor 18. The D/A convertor 18 convertsthe output digital code from the microcomputer circuit 16 into an analogvoltage to supply it to the voltage-controlled piezoelectric oscillator11.

With this operation, the output frequency F from the voltage-controlledpiezoelectric oscillator 11 can always be kept at the center frequencyf₀ regardless of a change in ambient temperature T. In this embodiment,the change amount Δf of the output frequency F from thevoltage-controlled piezoelectric oscillator 11 with respect to thecenter frequency f₀ is calculated on the basis of the frequencydifference between the first and second divided output frequencies F1and F2. Even if the ambient temperature T abruptly changes, the changeamount Δf of the output frequency F from the voltage-controlledpiezoelectric oscillator 11 can be instantaneously calculated toimmediately keep the output frequency F at the center frequency f₀.

As is apparent from the above description, according to the presentinvention, when the output frequency F from the voltage-controlledpiezoelectric oscillator changes upon a change in ambient temperature T,a difference between the first and second divided output frequencies F1and F2 also changes. On the basis of this difference between the firstand second divided output frequencies F1 and F2, the output frequency Ffrom the voltage-controlled piezoelectric oscillator is kept at thecenter frequency f₀. Since the change amount Δf of the output frequencyF from the voltage-controlled piezoelectric oscillator is detectedinstantaneously, accurate temperature compensation can be performed evenupon an abrupt change in ambient temperature T not to degrade frequencystability.

When the output frequency F from the voltage-controlled piezoelectricoscillator changes upon a change in ambient temperature T, a differencebetween the first and second divided output frequencies F1 and F2changes. The output frequency F from the voltage-controlledpiezoelectric oscillator is kept at the center frequency f₀ on the basisof a prestored reference value (a difference between the first dividedoutput frequency F1 (F1=f₀ /N) and the second divided output frequencyF2 (F2 =f₀ /M) at the ambient temperature T of t₀) and a differencebetween the first divided output frequency F1 (F1=(f₀ +Δf)/N) and thesecond divided output frequency F2 (F2=(f₀ +Δf)/M). Since the changeamount of the output frequency F from the voltage-controlledpiezoelectric oscillator is detected instantaneously, accuratetemperature compensation can be performed even upon an abrupt change inambient temperature T not to degrade frequency stability.

When the output frequency F from the voltage-controlled piezoelectricoscillator changes upon a change in ambient temperature T, a differencebetween the first and second divided output frequencies F1 and F2changes. The change amount Δf of the output frequency F from thevoltage-controlled piezoelectric oscillator with respect to the centerfrequency f₀ is calculated on the basis of a prestored reference value(a difference between the first divided output frequency F1 (F1=f₀ /N)and the second divided output frequency F2 (F2=f₀ /M) at the ambienttemperature T of t₀) and a difference between the first divided outputfrequency F1 (F1=(f₀ +Δf)/N) and the second divided output frequency F2(F2=(f₀ +Δf)/M). An analog voltage in accordance with this change amountΔf is applied to the voltage-controlled piezoelectric oscillator to keepthe output frequency F from the voltage-controlled piezoelectricoscillator at the center frequency f₀. Since the change amount Δf of theoutput frequency F from the voltage-controlled piezoelectric oscillatoris detected instantaneously, accurate temperature compensation can beperformed even upon an abrupt change in ambient temperature T not todegrade frequency stability.

What is claimed is:
 1. A temperature-compensated piezoelectricoscillator comprising:voltage-controlled piezoelectric oscillationmeans, having an output frequency controlled by a control voltage, foroscillating the output frequency having a center frequency f₀ at anambient temperature of t₀ ; first frequency division means forfrequency-dividing the output frequency from said voltage-controlledpiezoelectric oscillation means by N to output a first divided outputfrequency; second frequency division means for frequency-dividing theoutput frequency from said voltage-controlled piezoelectric oscillationmeans by M to output a second divided output frequency; and controlmeans for controlling the control voltage to be applied to saidvoltage-controlled piezoelectric oscillation means on the basis of afrequency difference between the first and second divided outputfrequencies upon a variation in ambient temperature of t₀ to keep theoutput frequency of said voltage-controlled piezoelectric oscillationmeans at the center frequency f₀.
 2. An oscillator according to claim 1,further comprising storage means for storing a frequency differencebetween the first and second divided output frequencies at the ambienttemperature of t₀ as a reference value, so that said control meanscalculates a value of the control voltage to be applied to saidvoltage-controlled piezoelectric oscillation means on the basis of thereference value read out from said storage means and the frequencydifference between the first and second divided output frequencies uponthe variation in ambient temperature of t₀.
 3. An oscillator accordingto claim 2, wherein said control means obtains a change amount Δf of theoutput frequency from said voltage-controlled piezoelectric oscillationmeans with respect to the center frequency f₀ to calculate the value ofthe control voltage in accordance with Δf(1/N-1/M).
 4. An oscillatoraccording to claim 1, further comprising frequency comparison means forcomparing the first and second divided output frequencies to each otherto obtain a frequency difference.
 5. An oscillator according to claim 4,further comprising an A/D convertor for converting an analog voltagerepresenting the frequency difference output from said frequencycomparison means into a digital code, and D/A conversion means forconverting a digital code representing the control voltage output fromsaid control means into an analog voltage, and wherein said controlmeans is constituted by a microcomputer.
 6. A temperature-compensatedpiezoelectric oscillator comprising:a voltage-controlled piezoelectricoscillator having an output frequency controlled by a control voltage tooscillate the output frequency having a center frequency f₀ at anambient temperature of t₀ ; a first frequency divider forfrequency-dividing the output frequency from said voltage-controlledpiezoelectric oscillator by N to output a first divided outputfrequency; a second frequency divider for frequency-dividing the outputfrequency from said voltage-controlled piezoelectric oscillator by M tooutput a second divided output frequency; a high-precision frequencycomparator for comparing the first and second divided output frequenciesto each other to output an analog voltage in accordance with a frequencydifference; an A/D converter for converting the analog voltage from saidhigh-precision frequency comparator into a digital code; a memory forstoring a frequency difference between the first and second dividedoutput frequencies at the ambient temperature of t₀ as a referencevalue; a microcomputer for obtaining a change amount of the outputfrequency from said voltage-controlled piezoelectric oscillator withrespect to the center frequency f₀ on the basis of the reference valueread from said memory and the digital code from said A/D convertor tooutput a digital code in accordance with the change amount; and a D/Aconvertor for converting the digital code output from said microcomputerinto an analog voltage to apply the analog voltage to saidvoltage-controlled piezoelectric oscillator as the control voltage,wherein said microcomputer outputs the control voltage to be applied tosaid voltage-controlled piezoelectric oscillator through said D/Aconvertor to keep the output frequency from said voltage-controlledpiezoelectric oscillator at the center frequency f₀.